372 



A MANUAL OF PHYSIOLOGY 



Pytor 



Plexus gast 

 anterior vag, 



Oesophagus 



us gas trie us 

 poster/or vagi. 



or ax on- reflex (p. 809), elicited by excitation of efferent fibres, 

 which send branches to some of the ganglion cells. 



The salivary centre can also be inhibited, especially by emotions 

 of a painful kind for instance, the nervousness which often 

 dries up the saliva, as well as the eloquence, of a beginner in 

 public speaking, and the fear which sometimes made the medieval 

 ordeal of the consecrated bread pick out the guilty. 



In rare cases the reflex nervous mechanism that governs the 

 salivary glands appears to completely break down ; and then 

 two opposite conditions may be seen xerostomia, or ' dry 

 mouth,' in which no saliva at all is secreted, and chronic ptyalism, 

 or hydrostomia, where, in the absence of any discoverable cause, 

 the amount of secretion is permanently increased. Both conditions 



are said to be 

 more common 

 in women than 

 in men. 



The Influ- 

 ence of Nerves 

 on the Gastric 

 Glands. Like 

 saliva, gastric 

 juice is not 

 secreted con- 

 tinuously, ex- 

 cept in ani- 

 mals such as 

 the rabbit, 

 whose stom- 

 achs are never 

 empty. The 

 normal and 

 most efficient 



stimulus is the eating of food and its presence in the stomach. 

 Mechanical stimulation of the gastric mucous membrane with 

 a non-digestible substance, such as a feather or a glass rod, 

 causes secretion of mucus, but not of gastric juice. But the 

 observations mentioned above on the difference of response 

 of the salivary glands to different substances suggest that 

 the local mechanical stimulation of the food on the gastric 

 glands may be more effective. There is also at first thought 

 much to indicate that the gastric glands are stimulated chemi- 

 cally in a more direct manner than the salivary glands by the 

 local action of food substances reaching the cells by a short- 

 cut from the cavity of the stomach, or in a more roundabout way 

 by the blood. And it might be very plausibly argued that the 



FIG. 144. PAWLOW'S STOMACH POUCH. 



AB, line of incision ; C, flap for forming the stomach pouch. 

 At the base of the flap the serous and muscular coats are 

 preserved, and only the mucous membrane divided, so that 

 the branches of the vagus going to the pouch are not severed. 



